1. Field of the Invention
The present invention relates to an image processing apparatus and color processing method and, more particularly, to an image processing apparatus and color processing method which correct image data based on the colorimetric values of patches.
2. Description of the Related Art
In general, the colorimetric values of a colored object change depending on temperature, and this phenomenon is called thermochromism. Thermochromism poses a problem in a situation that requires high-precision colorimetry since it causes errors of colorimetric values according to the temperature of an object to be measured. However, many things are still unknown about the relationship between thermochromism and colorimetry.
Color stabilization and color matching processing in a general printing apparatus will be described below. Conventionally, in order to control a printing apparatus represented by a printer apparatus to output desired colors, a color conversion lookup table (to be abbreviated as LUT hereinafter) is used. The color conversion LUT includes an LUT used in calibration required to maintain a printing apparatus in a constant state, and an LUT used in color matching represented by ICC profiles. In order to create these color conversion LUTs, for example, a printing apparatus outputs a plurality of colors of patches such as patches according to IT8 standard IT8.7/3. The colors of these plurality of colors of patches are measured using a colorimeter such as a spectral colorimeter to obtain colorimetric values to associate device values with device-independent values, thus creating a color conversion LUT. The created LUT absorbs a color appearance difference between devices or that due to variations of a printer engine, thus attaining color matching and color stabilization.
Recently, using a color sensor incorporated in a printer, this LUT is generated in real time. In such a printer, the built-in color sensor detects patches immediately after fixing, and feeds back their colorimetric values for LUT generation.
However, in a general printer, since the patches immediately after fixing have high temperatures (about 70° C.), the colorimetric values of the patches change due to the thermochromism. For this reason, an LUT suited to an observation environment (room temperature) of the user cannot be precisely created.
Therefore, a technique for correcting colorimetric values in consideration of the temperature at the time of colorimetry in a printer apparatus is demanded, and the following methods have been proposed.
In one method, spectral reflectance change amounts for respective wavelengths per unit temperature interval are calculated in advance for respective coloring samples, and the spectral reflectance at a desired temperature is predicted (for example, see patent reference 1).
In another method, change amounts of absorption coefficients and scattering coefficients in the Kubelka-Munk formula for respective wavelengths per unit temperature interval are calculated for respective coloring samples, and the spectral reflectance at a desired temperature is predicted (for example, see patent reference 2).    [Patent Reference 1] Japanese Patent Laid-Open No. 08-313353.    [Patent Reference 2] Japanese Patent Laid-Open No. 08-247932.
However, in the method which is described in patent reference 1 and calculates spectral reflectance change amounts for respective wavelengths per unit temperature interval, spectral reflectances for all combinations of device values that can be output by a printing apparatus cannot be predicted.
In the method which is described in patent reference 2 and calculates change amounts of absorption coefficients and scattering coefficients for respective wavelengths per unit temperature interval for respective coloring samples, the spectral reflectance of a mixed color can be predicted according to a mixing ratio of a coloring sample. However, since a printed material that has undergone general halftoning has an uneven colored surface, it is also difficult for this method to predict spectral reflectances for all combinations of device values that can be output by a printing apparatus.
Furthermore some print media used in general printing contain a fluorescent whitening agent, which absorbs ultraviolet rays and emits fluorescence in the visible range (especially, in a blue-violet range) so as to increase the degree of whiteness. Since the fluorescent whitening effect by the fluorescent whitening agent increases/decreases depending on temperatures, the colorimetric values of printed materials output onto media including the fluorescent whitening agent also change depending on temperatures.
Since neither of the two methods described in patent references 1 and 2 consider the influence of a fluorescent whitening agent contained in substrates of coloring samples (printing media), they also cannot appropriately correct colorimetric values for printed materials using media containing the fluorescent whitening agent.